1. Field of the Invention
The present invention relates to a thermal printhead for performing printing on a recording medium thermosensitively or by thermal transfer. The present invention also relates to a heating resistor used for a thermal printhead and a process of making a heating resistor.
2. Description of the Related Art
As is well known, a thermal printhead is used for selectively applying heat to a recording medium such as thermosensitive paper or a heat transfer ink ribbon to form necessary images. Thermal printheads are generally divided into thick-film thermal printheads and thin-film thermal printheads depending on the method of forming a heating resistor which generates heat when energized.
Referring to FIGS. 1 and 2, the structure of a prior art thermal printhead is described. The thermal printhead comprises a substrate 1 made of an alumina ceramic material for example and a glaze layer 2 for heat retention. The glaze layer 2 is formed with a common electrode 3 and individual electrodes 4 and is provided with a plurality of drive ICs 9 mounted thereon.
The common electrode 3 includes comb-teeth 5 and a common line 6 connecting the comb-teeth 5 to each other. Each of the individual electrodes 4 extends into a space between two adjacent comb-teeth 5. A heating resistor 7 is formed on and across the comb-teeth 5 and the individual electrodes 4. The heating resistor 7 is covered with a protective layer 8. The individual electrodes 4 are connected to the drive ICs 9 via wires 10 made of gold for example.
In the thermal printhead having the above-described structure, when a current is applied across two comb-teeth 5 between which an individual electrode 4 is arranged, part of the heating resistor 7 located between these two comb-teeth 5 is heated. As a result, one dot of an image is printed on a thermosensitive paper for example.
The heating resistor 7 contains a conductive substance such as ruthenium oxide and an insulating substance such as glass for example. Conventionally, for forming a heating resistor 7, a resistor paste is first prepared by mixing powder of a conductive substance with powder of glass, followed by further mixing therewith a resin, a solvent, and if necessary a filler for enabling printing. Subsequently, the resistor paste is applied on the substrate 1 into a strip. Then the resistor paste is dried, and baked at a temperature of about 810xc2x0 C. for example.
In forming the heating resistor 7, glass is mixed for the purpose of strongly bonding the heating resistor 7 onto the substrate 1 and for making the heating resistor 7 into a desired configuration. Moreover, by changing the mixing ratio of the glass to the conductive substance, it is possible to adjust the electric resistance of the heating resistor 7. The heating resistor 7 made by the above-described process has a structure wherein particles of the conductive substance are connected to each other in various directions so as to fill spaces between glass particles.
The electric resistance of the heating resistor 7 is determined in advance considering the use conditions of a thermal printhead, and the heating resistor 7 is so made as to provide the predetermined electric resistance. However, the voltage applied to the thermal printhead may vary depending on the kind and specifications of a printer, a facsimile machine or the like incorporating it. Therefore, in a thermal printhead, it is necessary to adjust the electric resistance in accordance with the voltage to be applied.
As described above, the adjustment of electric resistance is performed by changing the mixing ratio of glass and a conductive substance. For example, to decrease the electric resistance, the content of the conductive substance is increased. Conversely, to increase the electric resistance, the content of the conductive substance is decreased.
Generally, with respect to a heating resistor 7 of a thermal printhead, a heating resistor of a relatively high electric resistance (no less than 800xcexa9 for example) has a shorter life than a heating resistor of a relatively low electric resistance (less than 800xcexa9 for example) for the following reasons.
In a heating resistor with a low electric resistance, the content of the conductive substance is high so that many thick conductor paths are formed in a network fashion. When current flows through the low resistance heating resistor, electrons migrate between the particles of the conductive substance while generating heat. The heat melts the glass around the conductive substance particles, causing the so called xe2x80x9cthermal breakdownxe2x80x9d of the glass. In this way, a low resistance heating resistor breaks and ends its life mainly because of such thermal breakdown of the glass.
On the other hand, in a heating resistor with a high electric resistance, the content of the conductive substance is low so that the content of the glass is complementally high. Therefore, conductive paths are relatively thin and small in number. Therefore, even before the heating resistor is heated, the conductive path may be locally broken, which leads to breakdown of the heating resistor. For this reason, a high resistance heating resistor has a shorter life than a low resistance heating resistor of. Thus, there is a demand for a heating resistor which has a high electric resistance and also has a long life.
One of the operating characteristics of a heating resistor in a thermal printhead is endurable power. The endurable power indicates the magnitude of energy endured by a heating resistor when a current passes the heating resistor. For example, the endurable power may be expressed by the magnitude of electric energy at which the electric resistance of the heating resistor varies from its inherent value by no less than 15%. Therefore, a heating resistor having a greater endurable power provides less resistance variation and is therefore preferable for practical use.
The endurable power may be measured by a breakdown test called SST (step stress test). In this test, a pulse voltage of a given frequency is applied to a heating resistor while gradually changing the voltage as time elapses, wherein the change of the electric resistance is measured until the heating resistor breaks. While increasing the electric power in this test, the electric resistance once drops before the heating resistor breaks. This phenomenon is called xe2x80x9cminus driftxe2x80x9d. The minus drift occurs partly because of insufficient dispersion of the conductive substance in the heating resistor; i.e. the conductive substances are dispersed unevenly in the glass. The minus drift results in too black printing.
FIG. 3 illustrates the relationship between applied power and resistance variation ratio in SST when a heating resistor with a high sheet resistance of about 1.15 kxcexa9 (sheet resistance: resistance value for a square sheet having a thickness of 10 xcexcm and a side length of 3 mm) is activated for 32 dots. According to this figure, a minus drift occurs when the applied power becomes close to 0.8W. Such a minus drift is more likely to occur in a heating resistor having a lower content of conductive substance and hence a higher resistance than in a heating resistor with a higher content of conductive substance. This is because a heating resistor with a low electric resistance contains a large number of conductive particles, so that even if dispersion of the conductive particles is insufficient, it is possible to evenly disperse the conductive substance and glass. Therefore, a minus drift is less likely to occur.
On the other hand, in a heating resistor having a high electric resistance due to a low content of conductive substance, the number of conductive particles is small so that glass and the conductive substance cannot be easily dispersed evenly. For this reason, a minus drift is likely to occur. Therefore, there is a demand for a heating resistor which has a high electric resistance and is capable of preventing a minus drift.
It is therefore an object of the present invention to provide a heating resistor for a thermal printhead, which has a high resistance and a relatively long life but yet is capable of preventing minus drift.
Another object of the present invention is to provide a thermal printhead using such a heating resistor.
Still another object of the present invention is to provide a process of making such a heating resistor.
For achieving the objects described above, the present invention adopts the following measures.
In accordance with a first aspect of the present invention, there is provided a heating resistor for a thermal printhead, which contains a conductive substance and glass. The conductive substance is doped with an insulating substance which is identical in crystalline structure to the conductive substance.
Preferably, the conductive substance may comprise ruthenium oxide, and the insulating substance may comprise titanium oxide. Further, the content of titanium oxide in the titanium-oxide-doped ruthenium oxide may be 1-10 wt % (particularly 1-5 wt %).
In accordance with a second aspect of the present invention, there is provided a thermal printhead comprising a substrate, an electrode layer formed on the substrate, and a heating resistor which is formed on the electrode layer and contains a conductive substance and glass. The conductive substance is doped with an insulating substance which is identical in crystalline structure to the conductive substance.
In accordance with a third aspect of the present invention, there is provided a process of making a heating resistor for a thermal printhead comprising the steps of preparing a resistor paste by mixing powder of a conductive substance and glass powder, printing the resistor paste into a film, and baking the printed resistor paste. The method further comprises, before mixing the conductive substance powder with the glass powder, the steps of mixing powder of an insulating substance with the conductive substance powder, baking the obtained mixture, and pulverizing the baked mixture. The insulating substance is identical in crystalline structure to the conductive substance;
Preferably, the conductive substance comprises ruthenium oxide, whereas the insulating substance comprises titanium oxide. In this case, it is preferable that 1-10 wt % of titanium oxide is mixed with 99-90% of ruthenium oxide.
Other features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings.